Don't quit and return to the main menu when the credits start to roll in Assassin's Creed III -- there's an epilogue that continues the story and includes a trophy or achievement (worth 30 gamerscore).
The credits are ridiculously long, at least 10 minutes, and offer no option to skip them.
When they eventually finish, the epilogue will continue Connor's story and arguably set-up a sequel, after Sequence 12 concludes Desmond's tale.
There are a few missions, but they only serve to trigger cut-scenes -- there's no actual gameplay -- and then offer an excuse to keep the game playable with side quests once you've finished the main story.
By Ben Salter